The twenty millimeter ammunition currently made by all United States military services is primed by electricity and is employed in aircraft or other vehicle mounted guns where the vehicle's electric power is available to initiate the primer. In situations where infantry type weapons, particularly machine guns, are used but no such electric power source is available, limited-life power sources, e.g. batteries, can be used. These sources of power, because of their bulkiness and limited life, are not suitable for infantry use. This situation is undesirable and uneconomical from a military viewpoint because it otherwise necessitates the manufacture and use of the various types of ammunition of the same caliber employing percussion primers. The more widely used type of twenty millimeter ammunition is that which is electrically primed. There is a need, therefore, for an infantry type weapon, e.g. machine gun of the gas operated type, which has a compact, self-contained, low maintenance gun operated firing device for generating power to fire electrically primed ammunnition, which can be standardized and used in other applications.